The present invention relates to heat-sensitive recording paper, in particular, one having fast thermal response and which will cause reduced transfer of tailings to the thermal head.
Because of simplicity in the mechanism of color image formation by heating and the relative compactness of the recording apparatus with which it is used, heat-sensitive recording paper has been used extensively in various applications. As the use of heat-sensitive recording paper is expanding, the demand for reduced transfer of tailings to the thermal head and for higher sensitivity is also becoming more strict.
Conventional heat-sensitive recording paper has clay, talc and calcium carbonate incorporated in substrate paper. The substrate paper is weak in its ability to absorb the components of the heat-sensitive layer that melt upon heating and its capability of suppressing the transfer of tailings to the thermal head is almost nil. Thus, pigments capable of absorbing high oil contents have so far been incorporated in either the heat-sensitive color-forming layer or the undercoat layer or the substrate paper itself. For instance, Japanese Patent Publication No. 61-56118 and Japanese Patent Public Disclosure No. 59-155097 have proposed that pigments of high oil absorption be contained in the undercoat layer.
However, the use of such high oil-absorption pigments in the undercoat layer has the disadvantage that the components of the heat-sensitive color-forming layer that have been melted upon printing with high energy will be absorbed by the undercoat layer to cause occasional decrease in color density. To avoid this problem, it has been proposed that pigments of low oil absorption rather than high oil absorption be used in the undercoat layer (Japanese Patent Public Disclosure Nos. 61-139485 and 61-237683). However, if such low oil absorption pigments are used in the undercoat layer, the transfer of tailings to the thermal head will increase, thereby damaging the surface of heat-sensitive recording paper or causing printing defects such as low print density. Therefore, the previous attempts directed to the improvement of the undercoat layer, whether by incorporating high oil absorption pigments or by using low oil absorption pigments, have failed to attain heat-sensitive recording paper having good balance between sensitivity and resistance to transfer of tailings to the thermal head.
A method has also been proposed that relies upon incorporating high oil-absorption pigments in the substrate paper (Japanese Patent Public Disclosure No. 61-68291). However, this approach is not effective unless the pigments are loaded in large amounts but then the strength of the recording paper produced will inevitably decrease.